A large number of methods for producing N-phosphonomethylglycine are known. Some production methods result in the formation of an aqueous solution of an alkali metal salt and/or an alkaline earth metal salt of N-phosphonomethylglycine. Thus, a means for isolating and purifying N-phosphonomethylglycine from the aqueous solution in high purity and good yield has been in demand.
Japanese Unexamined Patent Publication No. 4-279595 discloses a method for producing N-phosphonomethylglycine which comprises reacting an aminomethylphosphonic acid and glycolonitrile in the presence of an alkali metal hydroxide. Then, an alkali metal hydroxide is further added in an amount sufficient to neutralize the carboxylic acid that is produced by the hydrolysis. According to this patent publication (Example 2), the reaction yield is very high, and the conversion ratio of each of the aminomethylphosphonic acid and glycolonitrile as raw materials is 95%. The purity is also high, and the reaction operation is simple. Hence, this is an excellent method for producing N-phosphonomethylglycine. However, the compound obtained after completing the reaction in this method is an alkali metal salt of N-phosphonomethylglycine. Therefore, N-phosphonomethylglycine must be isolated and purified therefrom by adding an acid. In the isolation and purification, means such as acid deposition or contacting with an ion exchange resin is used.
U.S. Pat. No. 4,221,583 describes in its Example a technique of using an ion exchange resin after hydrolyzing N-phosphonomethylglycinonitrile with hydrochloric acid or sodium hydroxide.
Thus, there is a need in the art for providing an improved method for isolating and purifying free N-phosphonomethylglycine from its alkali metal salt in high purity and good yield.